1. METAL CASTING PROCESSES
Sand Casting
Other Expendable Mold Casting Processes
Permanent Mold Casting Processes
Casting Quality
Metals for Casting
Product Design Considerations

2. Two Categories of Casting Processes
Expendable mold processes - mold is sacrificed to remove part
Advantage: more complex shapes possible
Disadvantage: production rates often limited by time to make mold rather than casting itself
Permanent mold processes - mold is made of metal and can be used to make many castings
Advantage: higher production rates
Disadvantage: geometries limited by need to open mold

3. Overview of Sand Casting
Most widely used casting process, accounting for a significant majority of total tonnage cast
Nearly all alloys can be sand casted, including metals with high melting temperatures, such as steel, nickel, and titanium
Castings range in size from small to very large
Production quantities from one to millions

4. Figure A large sand casting weighing over 680 kg (1500 lb) for an air compressor frame (photo courtesy of Elkhart Foundry).

5. Steps in Sand Casting Pour the molten metal into sand mold
Allow time for metal to solidify
Break up the mold to remove casting
Clean and inspect casting
Separate gating and riser system
Heat treatment of casting is sometimes required to improve metallurgical properties

6. Making the Sand Mold
The cavity in the sand mold is formed by packing sand around a pattern, then separating the mold into two halves and removing the pattern
The mold must also contain gating and riser system
If casting is to have internal surfaces, a core must be included in mold
A new sand mold must be made for each part produced

7. Sand Casting Production Sequence
Figure Steps in the production sequence in sand casting.
The steps include not only the casting operation but also pattern‑making and mold‑making.

8. Sand Casting

9. The Pattern
A full‑sized model of the part, slightly enlarged to account for shrinkage and machining allowances in the casting
Pattern materials:
Wood - common material because it is easy to work, but it warps
Metal - more expensive to make, but lasts much longer
Plastic - compromise between wood and metal

10. Types of Patterns Figure 11.3 Types of patterns used in sand casting:
(a) solid pattern
(b) split pattern
(c) match‑plate pattern
(d) cope and drag pattern

11. Core Full‑scale model of interior surfaces of part
It is inserted into the mold cavity prior to pouring
The molten metal flows and solidifies between the mold cavity and the core to form the casting's external and internal surfaces
May require supports to hold it in position in the mold cavity during pouring, called chaplets

12. Core in Mold
Figure (a) Core held in place in the mold cavity by chaplets, (b) possible chaplet design, (c) casting with internal cavity.

13. Desirable Mold Properties
Strength ‑ to maintain shape and resist erosion
Permeability ‑ to allow hot air and gases to pass through voids in sand
Thermal stability ‑ to resist cracking on contact with molten metal
Collapsibility ‑ ability to give way and allow casting to shrink without cracking the casting
Reusability ‑ can sand from broken mold be reused to make other molds?

14. Foundry Sands Silica (SiO2) or silica mixed with other minerals
Good refractory properties ‑ capacity to endure high temperatures
Small grain size yields better surface finish on the cast part
Large grain size is more permeable, allowing gases to escape during pouring
Irregular grain shapes strengthen molds due to interlocking, compared to round grains
Disadvantage: interlocking tends to reduce permeability

15. Binders Used with Foundry Sands
Sand is held together by a mixture of water and bonding clay
Typical mix: 90% sand, 3% water, and 7% clay
Other bonding agents also used in sand molds:
Organic resins (e g , phenolic resins)
Inorganic binders (e g , sodium silicate and phosphate)
Additives are sometimes combined with the mixture to increase strength and/or permeability

16. Types of Sand Mold
Green‑sand molds - mixture of sand, clay, and water;
“Green" means mold contains moisture at time of pouring
Dry‑sand mold - organic binders rather than clay
And mold is baked to improve strength
Skin‑dried mold - drying mold cavity surface of a green‑sand mold to a depth of 10 to 25 mm, using torches or heating lamps

17. Other Expendable Mold Processes
Shell Molding
Vacuum Molding
Expanded Polystyrene Process
Investment Casting
Plaster Mold and Ceramic Mold Casting

18. Shell Molding
Casting process in which the mold is a thin shell of sand held together by thermosetting resin binder
Figure Steps in shell‑molding: (1) a match‑plate or cope‑and‑drag metal pattern is heated and placed over a box containing sand mixed with thermosetting resin.

19. Shell Molding
Figure Steps in shell‑molding: (2) box is inverted so that sand and resin fall onto the hot pattern, causing a layer of the mixture to partially cure on the surface to form a hard shell; (3) box is repositioned so that loose uncured particles drop away;

20. Shell Molding
Figure Steps in shell‑molding: (4) sand shell is heated in oven for several minutes to complete curing; (5) shell mold is stripped from the pattern;

21. Shell Molding
Figure Steps in shell‑molding: (6) two halves of the shell mold are assembled, supported by sand or metal shot in a box, and pouring is accomplished; (7) the finished casting with sprue removed.

22. Advantages and Disadvantages
Advantages of shell molding:
Smoother cavity surface permits easier flow of molten metal and better surface finish
Good dimensional accuracy - machining often not required
Mold collapsibility minimizes cracks in casting
Can be mechanized for mass production
Disadvantages:
More expensive metal pattern
Difficult to justify for small quantities

23. Die Casting
A permanent mold casting process in which molten metal is injected into mold cavity under high pressure
Pressure is maintained during solidification, then mold is opened and part is removed
Molds in this casting operation are called dies; hence the name die casting
Use of high pressure to force metal into die cavity is what distinguishes this from other permanent mold processes

24. Die Casting Machines
Designed to hold and accurately close two mold halves and keep them closed while liquid metal is forced into cavity
Two main types:
Hot‑chamber machine
Cold‑chamber machine

25. Hot-Chamber Die Casting
Metal is melted in a container, and a piston injects liquid metal under high pressure into the die
High production rates parts per hour not uncommon
Applications limited to low melting‑point metals that do not chemically attack plunger and other mechanical components
Casting metals: zinc, tin, lead, and magnesium

26. Hot-Chamber Die Casting
Figure Cycle in hot‑chamber casting: (1) with die closed and plunger withdrawn, molten metal flows into the chamber

27. Hot-Chamber Die Casting
Figure Cycle in hot‑chamber casting: (2) plunger forces metal in chamber to flow into die, maintaining pressure during cooling and solidification.

28. Cold‑Chamber Die Casting Machine
Molten metal is poured into unheated chamber from external melting container, and a piston injects metal under high pressure into die cavity
High production but not usually as fast as hot‑chamber machines because of pouring step
Casting metals: aluminum, brass, and magnesium alloys
Advantages of hot‑chamber process favor its use on low melting‑point alloys (zinc, tin, lead)

29. Cold‑Chamber Die Casting
Figure Cycle in cold‑chamber casting: (1) with die closed and ram withdrawn, molten metal is poured into the chamber

30. Cold‑Chamber Die Casting
Figure Cycle in cold‑chamber casting: (2) ram forces metal to flow into die, maintaining pressure during cooling and solidification.

31. Molds for Die Casting
Usually made of tool steel, mold steel, or maraging steel
Tungsten and molybdenum (good refractory qualities) used to die cast steel and cast iron
Ejector pins required to remove part from die when it opens
Lubricants must be sprayed into cavities to prevent sticking

32. Advantages and Limitations
Advantages of die casting:
Economical for large production quantities
Good accuracy and surface finish
Thin sections are possible
Rapid cooling provides small grain size and good strength to casting
Disadvantages:
Generally limited to metals with low metal points
Part geometry must allow removal from die

33. Additional Steps After Solidification
Trimming
Removing the core
Surface cleaning
Inspection
Repair, if required
Heat treatment

34. Casting Quality
There are numerous opportunities for things to go wrong in a casting operation, resulting in quality defects in the product
The defects can be classified as follows:
General defects common to all casting processes
Defects related to sand casting process

35. General Defects: Misrun
A casting that has solidified before completely filling mold cavity
Figure Some common defects in castings: (a) misrun

36. General Defects: Cold Shut
Two portions of metal flow together but there is a lack of fusion due to premature freezing
Figure Some common defects in castings: (b) cold shut

37. General Defects: Cold Shot
Metal splatters during pouring and solid globules form and become entrapped in casting
Figure Some common defects in castings: (c) cold shot

38. General Defects: Shrinkage Cavity
Depression in surface or internal void caused by solidification shrinkage that restricts amount of molten metal available in last region to freeze
Figure Some common defects in castings: (d) shrinkage cavity

39. Metals for Casting
Most commercial castings are made of alloys rather than pure metals
Alloys are generally easier to cast, and properties of product are better
Casting alloys can be classified as:
Ferrous
Nonferrous

40. Product Design Considerations
Geometric simplicity:
Although casting can be used to produce complex part geometries, simplifying the part design usually improves castability
Avoiding unnecessary complexities:
Simplifies mold‑making
Reduces the need for cores
Improves the strength of the casting

41. Product Design Considerations
Corners on the casting:
Sharp corners and angles should be avoided, since they are sources of stress concentrations and may cause hot tearing and cracks
Generous fillets should be designed on inside corners and sharp edges should be blended

42. Product Design Considerations
Draft Guidelines:
In expendable mold casting, draft facilitates removal of pattern from mold
Draft = 1 for sand casting
In permanent mold casting, purpose is to aid in removal of the part from the mold
Draft = 2 to 3 for permanent mold processes
Similar tapers should be allowed if solid cores are used

43. Draft Minor changes in part design can reduce need for coring
Figure Design change to eliminate the need for using a core: (a) original design, and (b) redesign.

44. Product Design Considerations
Dimensional Tolerances and Surface Finish:
Significant differences in dimensional accuracies and finishes can be achieved in castings, depending on process:
Poor dimensional accuracies and finish for sand casting
Good dimensional accuracies and finish for die casting and investment casting

45. Product Design Considerations
Machining Allowances:
Almost all sand castings must be machined to achieve the required dimensions and part features
Additional material, called the machining allowance, is left on the casting in those surfaces where machining is necessary
Typical machining allowances for sand castings are around 1.5 and 3 mm (1/16 and 1/4 in)